This invention relates to saturated C.sub.1 -C.sub.8 alkyl (meth)acrylate oligomeric cosolvents for non-aqueous electrolytes. This invention also relates to a process for making saturated C.sub.1 -C.sub.8 alkyl (meth)acrylate oligomeric cosolvents for non-aqueous electrolytes. This invention further relates to an electrochemical cell having a cathode, an anode, and a nonaqueous electrolyte, in which the nonaqueous electrolyte contains a saturated C.sub.1 -C.sub.8 alkyl (meth)acrylate oligomeric cosolvent.
Nonaqueous batteries, particularly lithium batteries, have an excellent reputation for power output relative to size. A lithium cell requires the use of a nonaqueous electrolyte in view of the high chemical reactivity of lithium towards water. Certain low molecular weight diethers have been found to be very good solvents for use in electrolytes for lithium cells. For example, 1,2-dimethoxyethane is currently used in many lithium cells. Low molecular weight ethers are, however, relatively flammable having flash points less than 100.degree. C. Others have investigated the use of low molecular weight aliphatic esters such as methyl acetate and methyl formate but these have also been found to be highly flammable. For example, U.S. Pat. No. 4,804,596 discloses that lithium non-aqueous secondary electrochemical cells having a cathode active material having Li.sub.x CoO.sub.2 (0&lt;.times..ltoreq.1) preferably use electrolytes containing ester-based electrolyte solvents such as methyl acetate and methyl formate. However, methyl acetate and methyl formate are highly flammable, having flash point temperatures of -9.degree. C. and -26.degree. C., respectively, and normal boiling point temperatures of 57.degree. C. and 34.degree. C., respectively. As a result for safety concerns, electrochemical cells having electrolytes made with these solvents should generally be operated at temperatures less than 60.degree. C.
In order to improve a lithium cell, it is desirable to have a material with both a high flash point to reduce flammability and a high boiling point to improve the upper operating temperature limit of the cell.
U.S. Pat. Nos. 5,484,669 and 5,525,443 disclose the use of mixtures of cyclic esters (e.g., ethylene carbonate and propylene carbonate) with chain esters (e.g., diethyl carbonate, dimethyl carbonate, ethyl formate, methyl formate, ethyl acetate, methyl acetate, methyl butyrate, ethyl butyrate and ethyl isovalerate and dimethyl sulfoxide as nonaqueous electrolytes for lithium-ion batteries. All of these chain esters have molecular weights less than about 130 g/mol and flash points less than about 95.degree. C.
In U.S. Pat. No. 5,219,683 diesters and alkoxyalkylesters derived from diols have also been reported as solvents and cosolvents for nonaqueous battery electrolytes having high flash point temperatures. This patent discloses that ethylene glycol diacetate (flash point =82.degree. C.) and 2-ethoxyethyl acetate (flash point =57.degree. C.) are preferred when used alone or with propylene carbonate ("PC"). This patent also describes that diesters derived from two alcohols and a dicarboxylic acid have been reported as solvents or cosolvents for lithium batteries, but this patent describes that such diesters are not useful in lithium ion batteries.
It is also desirable to provide an efficient process for preparing saturated C.sub.1 -C.sub.8 alkyl (meth)acrylate oligomers having a molecular weight in the range of from about 160 to about 1000 g/mol as cosolvents for nonaqueous electrolytes. In the Journal of the American Chemical Society, 78, 472 (1956), Albisetti et al. describes the preparation of a saturated dimer of methyl methacrylate in a stainless steel (batch) autoclave heated under autogenous pressure at 225.degree. C. for 12 hours, followed by hydrogenation over palladium-on-charcoal catalyst. The duration of this process makes it inefficient and thereby unsuitable for commercial use.
In U.S. Pat. No. 4,546,160, Brand et al. describes a continuous bulk polymerization process for preparing unsaturated acrylic polymers having a molecular weight from about 700 to about 6000 g/mol. Brand discloses that an initiator must be used at reaction temperatures in the range of from 180.degree. C. to 270.degree. C. to prepare these polymers.
In accordance with the present invention, we have found that a family of saturated C.sub.1 -C.sub.8 alkyl (meth)acrylate oligomers possessing flash points greater than 100.degree. C. perform well as cosolvents for nonaqueous electrolytes. We have also found that these saturated oligomers are efficiently prepared by a process using a high temperature continuous flow stirred tank reactor ("CFSTR") that does not require an initiator. We have also found that lithium ion batteries having nonaqueous electrolytes containing saturated C.sub.1 -C.sub.8 alkyl (meth)acrylate oligomeric cosolvents have improved performance over similar batteries prepared with various other cosolvents.